Self-catalytic bath and method for the deposition of a nickel-phosphorus alloy on a substrate

ABSTRACT

Disclosed are a self-catalytic bath and a method for the deposition of Ni-P alloy on a substrate. The bath comprises nickel sulfate, sodium hypophosphite as a reducing agent, acetic acid as a buffer and traces of lead as a stabilizer. It also includes a citrate used as a complexing agent associated with a gluconate used both as a catalyst and a stabilizer. The disclosed bath makes it possible to tolerate large quantities of hypophosphite and is relatively long-lived. Furthermore, it can be used to prepare large quantities of Ni-P alloy per liter of solution.

This application is a continuation of application Ser. No. 08/954,193,filed Oct. 20, 1997, entitled SELF-CATALYTIC AND METHOD FOR THEDEPOSITION OF A NICKEL-PHOSPHORUS ALLOY ON A SUBSTRATE, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the formulation of a bath enabling thepreparation of a deposit of nickel-phosphorus (Ni-P) alloy on substratesused in the manufacture of printed circuit wafers and more particularly,to deposition of such alloys by chemical self-catalysis.

2. Discussion of the Related Art

Existing nickel baths are all relatively short-lived. Typical nickelbaths include combinations of complexing agents, buffers and stabilizersas well as nickel salt and phosphites. Typical complexing agents haveincluded lactic acid, maleic acid, aspartic acid, propionic acid,succinic acid, citric acid, etc. Several different complexing agents aregenerally associated with one another. It is known that the complexingagents, and especially citric acid or its derivatives, are incapable oftolerating large quantities of phosphites. The phosphite contentdetermines the quantity of phosphorus that can be deposited, and hencethe lifetime of the bath. Therefore, depletion of the bath's phosphitecontent decreases the bath lifetime.

Due to the short lifetimes of baths that presently exist in the market,it is very difficult to create conditions for making the deposits. It isknown that a small quantity of citric acid or of one of its derivativescauses a rapid decomposition of these baths by the precipitation ofnickel phosphites, therefore, citric acid and its derivatives are nottypically used as complexing agents. In addition, stabilizers arereputed to be poisons in the self-catalytic reaction, and thereforerequire handling with care. At present, the use of gluconates has beenlimited to cleansing solutions.

Therefore, due to the difficulties associated with creating theconditions necessary for making such deposits, the quality of thecoatings is reduced and their cost price remains very high.

SUMMARY OF THE INVENTION

The present invention provides a self-catalytic bath having a longerlifetime than presently existing baths. The bath according to thepresent invention includes nickel sulfate, sodium hypophosphite as areducing agent, acetic acid as a buffer, and traces of lead as astabilizer. More particularly, the bath includes a citrate complexingagent, associated with a gluconate used both as a catalyst and astabilizer. In preferred embodiments, the citrate is sodium tricitrateand the gluconate is sodium gluconate.

Advantages provided by the bath includes high quality deposits, veryhigh deposition rates, easy handling, and simple conditions ofimplementation. In addition, such deposits can also be made onnon-conductive substrates.

The bath according to the invention can tolerate large quantities ofhypophosphite, thereby considerably increasing its lifetime. When usedin continuous operation, the bath can provide about 38 grams of Ni-Palloy per liter of solution. Furthermore, the bath according to theinvention can be used to obtain amorphous deposits containing 13% byweight of phosphorus. Such deposits have a decorative appearance,greatly improved adhesion and hardness, and high corrosion resistance.

In another aspect, the present invention provides a method for thedeposition of Ni-P alloy on a substrate by self-catalytic chemicalmeans. The method involves plunging a substrate directly into the bathdescribed above. Preferably, the substrate is made of iron, mild steel,copper or again of a non-conductive material.

BRIEF DESCRIPTION OF THE DRAWINGS

Other particular features and advantages of the invention shall appearfrom the following description given by way of an illustrative andnon-restrictive example, with reference to the appended figures ofwhich:

FIG. 1 is a curve showing the influence of the pH of the solution on thespeed of deposition;

FIG. 2 is a curve showing the influence of the concentration in catalyston the speed of deposition;

FIG. 3 is a table showing all the values of the proportions ofconstituents needed to make a bath according to the invention.

DETAILED DESCRIPTION

The present invention provides formulations and methods for depositingnickel-phosphorus alloys. The bath formulation includes nickel sulfate,sodium hypophosphite as a reducing agent, acetic acid as a buffer,traces of lead as a stabilizer, and a citrate complexing agent,associated with a gluconate used both as a catalyst and a stabilizer.

According to the method, prior to deposition, it is necessary to carryout a surface treatment of the substrate in order to remove any surfaceoxide. Such surface treatment plays a major role, for an oxidizedsubstrate may lower the quality of the samples and pollute theelectrolyte. The surface treatment involves polishing the substratesmechanically 40 a grain size of 1,200, followed by degreasing withacetone and cleansing with a 10% sulfuric acid solution, and finally,rinsing with distilled water. The substrates used may, for example, bedesigned for the manufacture of printed circuit wafers, and maytypically be made of iron, copper, or mild steel. Of course, this listof materials is not exhaustive and other types of substrates may beused, including non-conductive substrates. The deposition ofnickel-phosphorus alloys on such substrates provides corrosionprotection, makes them suitable for welding, and gives a shinyappearance.

Copper substrates do not catalyze the deposition operation, andtherefore require a polarization of -1600 mV with respect to a referenceelectrode to be applied to it in order to initiate the self-catalyticprocess. Such an electrode is typically made of Ag-AgCl (a silverchloride electrode).

Several electrolytes presented hereinafter have been made and theirproperties have been compared in order to bring out the advantagesprovided by the bath according to the present invention. The deposits ofNi-P alloy were made on substrates of mild steel, iron or copper.

Coatings prepared in the presence of citrate as a complexing agent showhigher resistance to corrosion, which is why sodium tricitrate is used.Advantageously, the reagents used in the composition do not contain anychloride, for the Cl⁻ ions prompt a corrosion of the vessels that areused in industrial-scale manufacture.

The composition of a first electrolyte (hereinafter called "electrolyteI") enabling the preparation of the coatings of the alloy Ni-P, andwhich does not include a catalyst, was formulated to include nickelphosphate, sodium hypophosphite and sodium triacetate in the followingproportions:

0.1 M NiSO₄ :6H₂ O (nickel sulfate @ 26 g/l));

0.28 M NaH₂ PO₂ :H₂ O (sodium hypophosphite @ 24.6 g/l); and

0.2 M Na₃ C₆ H₅ O₇ :2H₂ O (sodium tricitrate @ 58.8 g/l).

The initial pH was set at 5.5 by the addition of an acetic acid buffer,preferably 13.6 ml of acetic acid, for a solution concentration of 0.024M CH₃ CO₂ H. The temperature was adjusted and maintained at T=87±2° C.,by placing the cell containing the bath into a thermostat-controlledbath. This formulation gives only about 0.5 g of Ni-P alloy per liter ofsolution, due to the fact that the use of sodium tricitrate at 0.2 Mleads to the rapid decomposition of the bath by the precipitation ofnickel phosphite after 10 minutes of operation.

The potential of deposition and the pH do not vary during theself-catalytic reaction. This is due to the fact that the buffer used,namely acetic acid, is of good quality. Furthermore, the curve of FIG.1, representing the effect of the pH on the speed of deposition, showsthat when the pH goes from 4 to 6, the speed of deposition of this bathincreases from 1 to 5.8 μm/h. This phenomenon can be explained by thefact that the value of the pH plays a major role in the reaction of theoxidation of hypophosphite (H₂ PO₂ ⁻). For this reason, the value of thepH of the electrolyte I has been set at 5.5.

The speed of deposition obtained is in the range of 5 μm/h, which isbelow the standard required industrially which is in the range of 8 to10 μm/h. Therefore, to improve the speed of deposition and the lifetimeof the bath, organic compounds have been added and certain operationalparameters have been modified. It has been found that the concentrationof complexing agent influences the depositions speed. Therefore, theconcentration of sodium citrate is reduced compared to electrolyte I. Ithas also been found that the addition of certain catalysts can stabilizethe bath and increase the deposition speed. Accordingly, the compositionof the second resultant electrolyte, called "electrolyte II",corresponding to the bath according to the invention, is as follows:

0.1 M NiSO₄ : 6H₂ O (nickel sulfate @ 26 g/l);

0.33 M NaH₂ PO₂ :H₂ O (sodium hypophosphite @ 29 g/l);

0.15 M Na₃ C₆ H₅ O₇ :2H₂ O (sodium tricitrate @ 44 g/l); and

0.092 M C₆ H₁₁ NaO₇ (sodium gluconate @ 20 g/l).

As previously described, the pH is set at 5.5 by the addition of aceticacid, which acts as a buffer. The addition of the complexing agentreduces the value of the concentration in free metal ions [M^(n+) ] byseveral magnitudes and thus reduces the potential E_(m), thus providinga possibility of controlling the kinetics during deposition, as setforth in the following equations.

The reduction of the metal ions is given by the reaction:

M^(n+) +ne⁻ →M°

The corresponding potential is written as follows:

Em=E°(M^(n+) /M)+2.3RT/nf log ([M^(n+) free]/[M]).

The choice of a gluconate as a catalyst can be explained by the factthat this type of compound contains hydroxide functions capable ofpicking up the H⁺ protons, thus preventing their adsorption on thesurface of the substrate and the blocking of the active sites. Thecatalysts eliminating ions likely to occupy active sites and thereforeaccelerate the deposition process. The use of sodium tricitrate withouta catalyst, specifically gluconate, leads to the decomposition of thebath after 10 minutes of operation. An initial interpretation of theseresults would be that it is the diminishing of the concentration incitrates that increases the speed of deposition while gluconate onlystabilizes the bath. However, it has been found that the depositionspeed varies greatly depending on the gluconate concentration. The curveof FIG. 2, indicating the variation of the speed of deposition as afunction of the gluconate concentration, has a shape which is typical ofthat of a catalyst. Consequently, the gluconate, in addition to itsstabilizing role, is used to substantially increase the speed ofdeposition. Indeed, FIG. 2 shows that the speed of deposition increaseswith the gluconate concentration to reach a maximum speed of 10 μm/h ata concentration of 20 g/l. This is the maximum concentration beyondwhich a slowing down of the kinetics is observed. The existence of amaximum speed shows that, for quantities greater than 20 g/l, thegluconate behaves like a complexing agent. This feature is probablyrelated to the stabilizing role of this type of compound.

The self-catalytic bath is preferably made with two solutions ofidentical volumes which are mixed in an electrolytic cell. In apreferred embodiment, a one liter bath prepared by mixing 500 ml offirst and second solution in an electolytic cell. Preferably, the firstsolution is prepared to include

0.1 M NiSO₄ : 6H₂ O (nickel sulfate @ 26 g/l);

0.15 M Na₃ C₆ H₅ O₇ :2H₂ O (sodium tricitrate @ 44.11 g/l), and

CH₃ CO₂ H 13.6 ml/l (acetic acid @ 13.6 ml/l).

The second solution is prepared to include:

0.33 M NaH₂ PO₂ :H₂ O (sodium hypophospite @ 29 g/l),

0.092 M C₆ H₁₁ NaO₇ (sodium gluconate @ 20 g/l), and

traces of lead.

To determine the lifetime of the bath and the weight of Ni-P alloy thatcan be prepared per liter of solution, the electrolyte must work and beregenerated continuously. The determining of the composition of theregeneration solutions makes it necessary to take account of thestability of the bath, the speed of deposition and the appearance of thecoatings.

The regeneration solution of the bath according to the invention hasbeen made in a cell with a effective volume equal to one liter inkeeping a ratio of "surface of the substrate" to "volume of electrolyte"equal to about 120 cm² /l. This solution is prepared out of nickelsulfate, sodium tricitrate and sodium hypophosphite in the followingproportions, and is shaken mechanically:

0.59 M NiSO₄ :6H₂ O (nickel sulfate @ 155 g/l);

0.12 M Na₃ C₆ H₅ O₇ :2H₂ O (sodium tricitrate @ 35 g/l); and

2.27 M NaH₂ PO₂ :H₂ O (sodium hypophosphite @ 200 g/l).

Another ammonia solution diluted to 10% is furthermore used tocompensate for the diminishing of the pH due to the oxidation of thehypophosphite and the reduction of the nickel ions during theregeneration of the bath. The quantities of constituents needed toprepare the bath, regenerate it, and deposit one gram of Ni-P alloy areassembled in the table of FIG. 3.

The regeneration of the bath is done manually, in the course of acontinuous quantitative analysis of the bath, to recover the initialconcentration of the different constituent elements. The addition isdone directly after the consumption of 10% of Ni²⁺ ions, typically afterabout 40 minutes. The pH factor is set at 5.5±0.2 and readjusted bymeans of the ammonia solution at 10%, after every 30 minutes. The bathis kept at constant temperature, equal to 87° C.±2° C. For this purpose,the cell containing the bath is plunged into a thermostat-controlledbath.

This formulation has made it possible to carry out six "turnovers" witha mean speed of 12 μm/h. One turnover corresponds to the consumption of6 grams of Ni²⁺ ions per liter of solution. The coatings keep theirshiny appearance throughout the regeneration of the bath.

The addition of ammonia solution in a concentration of 10% is done tocompensate for the reduction over time, of the pH of the solution.Indeed, the reaction of the oxidation of hypophosphite generates H⁺protons according to the following reaction:

    H.sub.2 PO.sub.2.sup.- +H.sub.2 O→H.sub.2 PO.sub.3.sup.- +2H.sup.+ +2e.sup.-                                                 (1)

Furthermore, the reduction of the Ni²⁺ ions may also release protonsaccording to the reaction:

    Ni.sup.2+ +2H→Ni+2H.sup.+                           (2)

and therefore also participate in the acidification of the medium.

The incorporation of phosphorus is done according to the followingreaction:

    H.sub.2 PO.sub.2.sup.- +H.sub.ads →P+OH.sup.- +H.sub.2 O (3)

where H_(ads) represents hydrogen adsorbed into the surface of thesubstrate. This last-named reaction (3) has an effect opposite that ofthe first two reactions (1) and (2) but its contribution remains lowsince the phosphorus content does not exceed 22% in atoms.

In the present state of knowledge, the baths based on citric acid and/orits derivatives are known to be incapable of tolerating large quantitiesof phosphites. In the case of continuous operation, the values ofphosphite content are indeed generally in the range of 0.03 M for valuesof content of complexing agent in the range of 0.07 M. Furthermore, theratio of the concentration in phosphites to the concentration in Ni²⁺ions is of the order of 0.3 to 0.4. These baths therefore have a limitedlifetime and can be used to obtain only about 5 g of Ni-P alloy perliter of solution.

The present invention makes it possible to prove the contrary since, inthe case of continuous operation, namely when it is regeneratedcontinuously, the bath disclosed herein tolerates a quantity ofhypophosphite of about 229 g/l for a quantity of sodium tricitrate of 79g/l. The ratio of the concentration in sodium hypophosphite to theconcentration in Ni²⁺ ions may therefore reach a value of 3 to 4.Furthermore, it is noted that gluconate also stabilizes the bath.Consequently, it is the joint action of citrate and gluconate thatenables the bath according to the invention to tolerate hypophosphiteconcentrations of this kind, yield a large quantity of Ni-P alloy perliter of solution and be long-lived.

The properties of the standard electrolytes of the prior art and of theelectrolytes I and II have been studied and compared. The results ofthese comparisons are assembled in the following table:

    ______________________________________                                                  Electrolyte I                                                                          Electrolyte II                                                                           Prior Art                                       ______________________________________                                        Concentration of                                                                          0.1 M      0.1 M      0.1 M                                         nickel salt (26 g/l) (26 g/l) (26 g/l)                                        Concentration of 0.1.M 0.1M 0.1 M                                             Ni.sup.2+ ions (6 g/l) (6 g/l) (6 g/l)                                        Concentration of 0.28 M 0.33 M 0.33 M                                         hypophosphite (24.6 g/l) (29 g/l) (29 g/l)                                    Concentration of 0.2 M 0.15 M citric acid                                     citrates (58.8 g/l) (44 g/l) 0.07 M                                              (13.4 g/l)                                                                 Quantities of 0 0.092 M 0                                                     gluconate  (20 g/l)                                                           Weight of Ni--P 0.5 g/l 38 g/l 5 g/l                                          alloy per                                                                     liter of solution (after                                                      regeneration)                                                                 (lifetime) (10 minutes) (7 hours) (4 hours)                                   % by weight of P in 5% 13% 10-11%                                             the deposited alloy                                                           pH 5.5 5.5 4.5 to 5.5                                                         T° 87° C. ± 2° C. 87° C. ± 2°                                        C. 70 to 100° C.                       Speed of 5 μm/h 12 μm/h 10-12 μm/h                                   deposition                                                                  ______________________________________                                    

These results clearly show that the electrolyte II according to theinvention has a greater lifetime and can be used to make a largequantity of Ni-P alloy per liter of solution.

The present invention will be further illustrated by the followingexample, which is intended to be illustrative in nature and is not to beconstrued as limiting the scope of the invention.

EXAMPLE

To measure the resistance to corrosion, trials were carried out in amedium with 3% NaCl and this resistance was assessed by measuring theresistance to polarization by electrochemical impedance. The samplestested were substrates on which deposits of Ni-P with a thicknessvarying from 6 to 8 μm had been made. The composition of Ni-P alloy wasdetermined by dispersion of energy×and the surface condition wasobserved by means of a scanning electron microscope.

The deposit contains 13% by weight of phosphorus and is homogenous. Thealloy has a nodular appearance and has no cracks. Observation with thenaked eye shows that the coatings are shiny and less uneven than thoseobtained with the presently used techniques. Their hardness is in therange of 550 Hv.

The films of Ni-P alloy deposited are amorphous and constitute anon-crystalline matrix in which crystalline nickel inclusionsover-saturated with phosphorus are incorporated. The high phosphoruscontent limits the quantity of these inclusions which are closelyrelated to the capacity of the film to corrode, confirming the excellentresistance to deterioration of the coatings obtained by means of thebath according to the invention. Indeed, the resistance to polarization,which is inversely proportional to the speed of corrosion measured onthe steel electrode coated with a layer of Ni-P, is very high, in therange of 10,000 W cm². Consequently, the Ni-P alloys obtained have highresistance to corrosion. This clearly matches the fact that these alloyshave great homogeneity.

To test the adhesion, the method involves folding the substrate coveredwith the Ni-P alloy to an acute angle, and then examining the adhesionat the deformed place. In all the samples tested, the Ni-P layer hashigh adhesion, which may also be seen during mechanical polishing, whenit is difficult to remove the Ni-P layers from the surface of thesubstrate.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be within the spirit andscope of the invention. Accordingly, the foregoing description is by wayof example only and is not intended as limiting. The invention islimited only as defined in the following claims and the equivalentsthereto.

What is claimed is:
 1. A self-catalytic bath comprising:nickel sulfate;sodium hypophosphite as a reducing agent; acetic acid as a buffer;traces of lead as a stabilizer; and a citrate used as a complexing agentand is associated with a gluconate that is used both as a catalyst and astabilizer.
 2. The bath according to claim 1, wherein the citrate issodium tricitrate and the gluconate is sodium gluconate.
 3. The bathaccording to claim 2, wherein, for a quantity of sodium tricitrate equalto 79 g/l, the bath includes about 229 g/l of sodium hypophosphite. 4.The bath according to claim 2, comprising;about 0.1 M NiSO₄ : 6H₂ O;about 0.15 M Na₃ C₆ H₅ O₇ :2H₂ O; about 0.33 M NaH₂ PO₂ :H₂ O; about0.092 M C₆ H₁₁ NaO₇ ; about 0.024 M CH₃ CO₂ H; traces of lead; andwherein the pH is set at about 5.5 and the temperature is set at about85 to about 89° C.
 5. The bath according to claim 3, comprising;about0.1 M NiSO₄ : 6H₂ O; about 0.15 M Na₃ C₆ H₅ O₇ :2H₂ O; about 0.024 M CH₃CO₂ H; about 0.33 M NaH₂ PO₂ :H₂ O; about 0.092 M C₆ H₁₁ NaO₇ ; tracesof lead; and wherein the pH is set at about 5.5 and the temperature isset at about 85 to about 89° C.
 6. The bath according to claim 1,further comprising a deposition rate of at least about 12 μm/h.
 7. Thebath according to claim 6 providing deposited coatings having shinyappearance.
 8. A bath according to claim 1 including a concentration ofsodium hypophosphite to a concentration of Ni²⁺ ions from nickel sulfatehaving a ratio of up to 3 to
 4. 9. A self-catalytic bathcomprising:nickel sulfate; sodium hypophosphite as a reducing agent;acetic acid as a buffer; traces of lead as a stabilizer; sodiumgluconate as a catalyst and a stabilizer; and complexing agent selectedto reduce a concentration of free metal ions in the bath by severalmagnitudes and thus reduce electric potential in order to controlkinetics of deposition of a nickel-phosphorous alloy.
 10. A bathaccording to claim 9 wherein said complexing agent is sodium tricitrate.11. A bath according to claim 10 wherein said sodium gluconate bas aconcentration of up to 20 g/l.
 12. A bath according to claim 10 whereinthe pH is set at about 5.5 and the temperature is set at about 85° C. toabout 89° C.
 13. A self-catalytic bat comprising:nickel sulfate; sodiumhypophosphite as a reducing agent; acetic acid as a buffer; traces oflead as a stabilizer; a citrate used as a complexing agent; and acatalyst and a stabilizer, said catalyst including hydroxide functionscapable of picking up H⁺ protons, thus preventing adsorption of the H⁺protons on the surface of a substrate during deposition of anickel-phosphorous alloy, said stabilizer preventing decomposition ofthe bath and increasing deposition of said nickel-phosphorous alloy. 14.A bath according to claim 13 wherein said catalyst and said stabilizeris sodium gluconate.
 15. A bath according to claim 14 wherein saidsodium gluconate has a concentration of up to 20 g/l.
 16. A bathaccording to claim 14 wherein the pH is set at about 5.5 and thetemperature is set at about 85° C to about 89° C.